Many studies have reported on the different types of placenta in different species, but accessory or paraplacental structures are mentioned only in species that have such a nutrient-exchanging mechanism. Hystricomorph rodents possess a subplacenta, swine and equines have the placental areola, equines have the chorionic girdle, and carnivores have hemophagous organs that are equivalent to the placental hematoma in ruminants. These accessory structures are specialized tissues for specific nutrient exchanges in different species, and they are adaptive modifications exhibited by domestic and wild species to remedy the nutritional deficiencies that are related to permeability of the main placenta to important nutrients for the growth and development of the fetus during the gestation.
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